1
25-074
Preventing Stope Back Collapse—Practical Strategies
Robert Cook
Call &Nicholas, Inc., Tucson, AZ
Jason Hallowell
Call &Nicholas, Inc., Tucson, AZ
Valeria Zarate
Call &Nicholas, Inc., Tucson, AZ
ABSTRACT
The long hole open stoping (LHOS) method is widely used
in underground mining. While methods to predict stable
stope geometries are well established, the ability to main-
tain a stable back often assumes effective ground support
across the full stope span. The efficacy of the support strat-
egy is dependent on rock jointing, quality, density, top cut
width and height, and the type, amount, and orientations
of bolts installed. However, it is becoming increasingly
common for operators to assume narrow top cuts in service
of wider stope spans with minimal bolting coverage. Failure
to account for the required top cut drift size and support
requirements can ultimately lead to raveling type instabili-
ties along stope shoulders that result in back collapse. A
new method has been developed to evaluate the minimum
top cut dimension necessary to accommodate wider stope
spans with various rock bolting configurations. Due to the
economic impacts of top cut development and support,
this is a critical consideration in the accurate costing and
scheduling of a LHOS operation.
Tables 1 lists the acronyms are used throughout this
paper.
INTRODUCTION
While the prediction of stable stope geometries is straight-
forward using the Stability Graph Method (Mathews
et al., 1980), the ability to maintain a stable back often
assumes effective ground support across the entire stope
span. During early design stages of mine feasibility studies,
it is common to assign a stope top cut a nominal width,
without any consideration of the ground support require-
ments necessary to manage back stability. These widths will
control the development rates and tonnages associated with
operational costs. However, depending on the site condi-
tions, a unique top cut width and secondary ground sup-
port system (long bolts) may be necessary to prevent back
collapse. Because wider top cuts will impact the amount of
drift development relative to production tons, and because
ground support can be costly and time consuming to
install, this is a critical consideration in the accurate costing
and scheduling of a LHOS operation.
Using both stress modeling and kinematic analyses
based on discrete fracture network models, a method has
been developed to evaluate the minimum top cut dimen-
sion necessary to accommodate wider stope spans with
various rock bolting configurations.
STOPE STABILITY
The Stability Graph Method as modified by Potvin (1988)
and Nickson (1992) is the industry standard methodol-
ogy for determining stope dimensions. This method is an
empirical design tool based on case histories from hard
rock mines which typically have good to very good qual-
ity rock. It accounts for key factors influencing open stope
design, including rock-mass strength and structure, stresses
surrounding the opening, and the shape and orientation
of the stope. Based on a calculated stability number, N’,
hydraulic radii of stope walls can be predicted using the
design curves. However, there exists separate design curves
for both unsupported cases and supported cases. In most
cases, stope walls (side and end) are designed using the
design curve from unsupported case histories, whereas the
back, which has top cut access so that bolts can be installed
Table 1. List of Acronyms
Bottom Cut BC
Discrete Fracture Network DFN
Fall of Ground FOG
Factor of Safety FOS
Long Hole Open Stoping LHOS
Quality Assurance /Quality Control QA/QC
Top Cut TC
25-074
Preventing Stope Back Collapse—Practical Strategies
Robert Cook
Call &Nicholas, Inc., Tucson, AZ
Jason Hallowell
Call &Nicholas, Inc., Tucson, AZ
Valeria Zarate
Call &Nicholas, Inc., Tucson, AZ
ABSTRACT
The long hole open stoping (LHOS) method is widely used
in underground mining. While methods to predict stable
stope geometries are well established, the ability to main-
tain a stable back often assumes effective ground support
across the full stope span. The efficacy of the support strat-
egy is dependent on rock jointing, quality, density, top cut
width and height, and the type, amount, and orientations
of bolts installed. However, it is becoming increasingly
common for operators to assume narrow top cuts in service
of wider stope spans with minimal bolting coverage. Failure
to account for the required top cut drift size and support
requirements can ultimately lead to raveling type instabili-
ties along stope shoulders that result in back collapse. A
new method has been developed to evaluate the minimum
top cut dimension necessary to accommodate wider stope
spans with various rock bolting configurations. Due to the
economic impacts of top cut development and support,
this is a critical consideration in the accurate costing and
scheduling of a LHOS operation.
Tables 1 lists the acronyms are used throughout this
paper.
INTRODUCTION
While the prediction of stable stope geometries is straight-
forward using the Stability Graph Method (Mathews
et al., 1980), the ability to maintain a stable back often
assumes effective ground support across the entire stope
span. During early design stages of mine feasibility studies,
it is common to assign a stope top cut a nominal width,
without any consideration of the ground support require-
ments necessary to manage back stability. These widths will
control the development rates and tonnages associated with
operational costs. However, depending on the site condi-
tions, a unique top cut width and secondary ground sup-
port system (long bolts) may be necessary to prevent back
collapse. Because wider top cuts will impact the amount of
drift development relative to production tons, and because
ground support can be costly and time consuming to
install, this is a critical consideration in the accurate costing
and scheduling of a LHOS operation.
Using both stress modeling and kinematic analyses
based on discrete fracture network models, a method has
been developed to evaluate the minimum top cut dimen-
sion necessary to accommodate wider stope spans with
various rock bolting configurations.
STOPE STABILITY
The Stability Graph Method as modified by Potvin (1988)
and Nickson (1992) is the industry standard methodol-
ogy for determining stope dimensions. This method is an
empirical design tool based on case histories from hard
rock mines which typically have good to very good qual-
ity rock. It accounts for key factors influencing open stope
design, including rock-mass strength and structure, stresses
surrounding the opening, and the shape and orientation
of the stope. Based on a calculated stability number, N’,
hydraulic radii of stope walls can be predicted using the
design curves. However, there exists separate design curves
for both unsupported cases and supported cases. In most
cases, stope walls (side and end) are designed using the
design curve from unsupported case histories, whereas the
back, which has top cut access so that bolts can be installed
Table 1. List of Acronyms
Bottom Cut BC
Discrete Fracture Network DFN
Fall of Ground FOG
Factor of Safety FOS
Long Hole Open Stoping LHOS
Quality Assurance /Quality Control QA/QC
Top Cut TC